A number of industrial and military processes require particulate to be removed from an aerosol apparatus and deposited onto a surface. Two examples are electrostatic powder painting and particle concentrators, which are components of chemical and biological detection systems. The importance of electrostatics for this purpose is well known to those of skill in the art.
The use of electrostatics-based systems as a means of removing particulate from an aerosol has been known for over seventy years. The first practical use of electrostatics-based systems for this purpose was the electrostatic precipitator used to clean the exhaust systems in various industrial settings, including power generating plants, chemical processing plants and pharmaceutical plants. These early electrostatic precipitators, still used to achieve the particulate removal, are characterized by very simple construction and operating principles. Most consist of a wire concentrically positioned at the center of a cylindrical duct and a high voltage applied to a central conductor sufficient to produce a corona current between the wire and the duct wall. The corona produces a unipolar charge density between the wire and the duct walls. Particulate entering the corona field charges according to the field charging equations described by Pauthenier, which are well-known to those of skill in the art, and is then forced to the duct wall by the electric field applied between the wire and the duct wall.
Thirty years after the commercial development of the electrostatic precipitator, a second commercial application of electrostatics was developed: electrostatic particulate deposition. This time the application was in the area of industrial powder painting. The primary industrial advantage of applying paint coatings as a powder is the removal of solvents from the painting process. These industrial powder coating systems operate in a manner very similar to that described for electrostatic precipitators.
The main difference between the two systems is the manner in which the corona ion current is developed and used. The powder coating systems use one or more electrodes placed at the output of an insulating tube through which powder and air are conveyed. The electrode or electrodes are electrically biased to a voltage sufficient to create a corona current between the electrodes and a grounded deposition surface. The ion flux flowing between the electrodes and the deposition surface charge the particles leaving the tube. The charged particulate is then conveyed to the deposition surface by the forces applied from both the electric field and the aerodynamic drag generated by the conveying air. Deposited particles adhere to the deposition surface due to electrostatic forces formed between the particulate matter and the grounded surface as well as to Van der Waals forces.
A disadvantage of the industrial systems described above is that the charge density and the electric field within the particulate charging zone are non-uniform. It is well documented that current corona wire charging systems produce spatially varying corona current density and electric field along their axial dimension. This effect causes these systems to be much larger than is necessary to meet the requirements for particulate removal. This geometry also forces the deposition of the particulate onto the cylindrical duct surface. For systems needing focused, efficient particulate concentration, this geometry is particularly unattractive.
One example from the prior art that demonstrates this problem is the electrostatic spray gun used for powder coating. This device uses single or multiple electrodes arranged at the output of a cylindrical tube having a diameter of about ⅝″. The target deposition surface is usually 12-24″ from the point or points of the corona ion current generation that occurs at the corona electrode. In this configuration, the corona ion current, whether generated from a single point or from multiple points, behaves very much like a point-to-plane corona ion current where the ion current is known to decay rapidly when measured at angles varying from normal to the deposition surface. Powder particle trajectories leaving the tube often fall outside the charging zone produced by this corona configuration. This results in a lowering of the transfer efficiency for the coating system.
In summary, there remains a need for more predictable and efficient corona particulate charging and deposition systems, especially for systems designed to focus and concentrate the particulate depositions. In the embodiments of the present invention, methods for more efficient corona particulate charging and deposition systems are shown. Likewise, it is important to develop a corona particulate charging system that dispenses with the need for a corona wire component. Hence, further advantages of embodiments of the present invention include the elimination of the need to accommodate cumbersome corona wire charging systems by eliminating the need for the corona wire component.
Embodiments of the present invention provide improved particulate deposition efficiency, spatial uniformity of depositions, and spatially-selective controlled depositions for the various particle transport systems. Embodiments of the present invention also provide new applications by the novel configuration and control of corona electrode arrays.